BACKGROUND: The role of maternal anaemia in preterm birth remains poorly defined, and the association between anaemia and preterm birth clinical subtypes remain unclear. We examined if maternal anaemia exposure both within and across trimesters during gestation is associated with preterm birth. METHODS: This was a secondary analysis of data from a population-based prospective cohort study in 13 counties of East China (1993-96). All singleton live births delivered at 20-44 weeks to women with at least one haemoglobin measure during pregnancy were included (n = 160 700). Risk of preterm birth (<37 weeks) was examined by clinical subtypes, namely, preterm premature rupture of membranes (PROM), spontaneous preterm labour and medically indicated preterm birth. Haemoglobin changes across trimesters were assessed as proxy of haemo-dilution and haemo-concentration. Multivariable Cox proportional hazards regression models were fitted. RESULTS: Preterm birth rates of preterm birth were 4.1% for anaemic and 5% for non-anaemic women (P < 0.05). Compared with haemoglobin of 11 g/dl (reference), values <or=5 g/dl in the first trimester were associated with increased risk for preterm PROM [hazard ratio (HR) 3.3, 95% confidence interval (CI) 1.4-7.7], whereas low haemoglobin in the third trimester was associated with reduced risk of spontaneous preterm labour. Haemodilution was associated with reduced risk for preterm birth. CONCLUSIONS: Anaemia in early pregnancy was found to be associated with increased risk for preterm PROM, whereas exposure in late pregnancy was associated with reduced risk for spontaneous preterm labour.